Abstract : Refractive index changes are the building blocks of laser-induced optical functions in bulk transparent materials, for example, in fused silica. Depending on the regime of laser interaction, focused ultrashort pulses could induce either positive or negative isotropic refractive index changes (usually denoted as type I) or produce nanoscale self-arranging layered structure resulting in form birefringence (type II regime) [1]. As these types of refractive index changes have consequences in the functionality and performances of 3D optical devices (e. g. embedded waveguides), an investigation of the laser-induced structures is particularly useful [2]. We propose Photoluminescence (PLM) and Raman microscopy (RM) to characterize refractive index changes and to provide insights into the responsible mechanisms. Revealed by PLM, electronic changes in the form of defects appearing in the fused silica, alters its electronic structure, and consequently its optical constants. Alternatively RM allows observing reorganizations of fused silica network and, connected to it, changes in density and refractive index. We equally explore primary stages of birefringent regions where we observe that nanoscale reorganization is preceded by voids formation due to self-focusing effects.. The accumulation of microexplosions leads finally to a regular periodic alternance of high and low density layers. Therefore, investigation of single and multiple pulse irradiation effects allows to emphasize several key factors defining laser modification, particularly its dependence on the energetic dose, pulse duration and focusing conditions. The proposed spectroscopy study distinguishes type I and type II regions by and NBOHC (?) defect centers formation and distribution. Taking into account that one of the mechanisms of defect generation is the liberation of molecular oxygen from the silica network, the presence of these defects in the regions of oxygen deficiency [1] could be interpreted as a result of oxygen migration during interaction. RM reveals signs of densification in the region of refractive index changes. At the same time zones with high concentration of non-bridging oxygen hole centers (NBOHC) with no visible refractive index changes and densification signs were detected. We propose that this defect generation occurs as a result of bond breaking after self-trapped exciton relaxation. If this process is efficient enough, non-bridging atoms favored by local increase of temperature could create new bondings forming network, corresponding to compacted silica. This investigation provides, thus, key elements in further elucidating the nature of ultrafast laser induced changes in the bulk of dielectrics. References. 1. R. Taylor, C. Hnatovsky, and E. Simova, "Applications of femtosecond laser induced self-organized planar nanocracks inside fused silica glass," Laser and Photon. Rev. 2, 26-46 (2008) 2. K. Mishchik et al., "Nanosize structural modifications with polarization functions in ultrafast laser-irradiated bulk fused silica," Optics Express, 18, 24809-24824, (2010).